Many valve guides for intake valves or exhaust valves in internal combustion engines are made of ferrous sintered alloys. For example, a sintered alloy in which carbon is 1.5 to 4 mass %, copper is 1 to 5 mass %, tin is 0.1 to 2 mass %, phosphorus is 0.1 to 0.3 mass % and the balance is iron, having Fe—C—P ternary eutectic compound precipitates in an iron pearlite matrix, and in which graphite is dispersed, is described in Japanese Patent Application Publication No. 55-34858. Since this alloy has superior machinability and wear resistance, it is used in engines such as those of automobiles. Corrosion-resistant and heat-resistant superalloy (Japanese Industrial Standard (JIS) NCF), heat-resistant steel (JIS SUH), high speed tool steel (JIS SKH), and alloys obtained by treating a soft nitriding process on the above-mentioned alloys are used for many valve stems in intake valves and exhaust valves.
It is desirable for the valve stem to be subjected to a soft nitriding processing treatment in order to improve thermostability, wear resistance and fatigue properties. However, this processing requires special management for handling and disposal, etc., because a melted salt including a toxic cyanogen compound is used, and there is a problem of environmental protection in this process. Therefore, valve stem without the soft nitriding process is desirable, if possible. However, in the case of using a valve stem without the soft nitriding process, sliding damage (abrasion) is progressive for valve guides made of ferrous sintered alloy and the valve stems, and there is a possibility that abrasion would relatively increase because wear resistance in the valve stem without the soft nitriding process is relatively lower than that in the valve stem treated the soft nitriding process. In particular, in the intake side (intake), the phenomenon called “oil dropping” in which the lubricating oil flows downward into the combustion chamber, occurs, when lubricating oil is supplied in order to prevent the abrasion of the slide part between the valve guide and the valve stem. By the phenomenon, the amount of consumption of the lubricating oil increases and the adverse emission effects are generated. Therefore, porosity of sintered alloy and feed rate of lubricating oil may be adjusted so that the quantity of lubricating oil becomes moderate, in order to avoid this generation. As a result, friction conditions become relatively severe.